Vehicles may be fitted with evaporative emission control systems such as onboard fuel vapor recovery systems. Such systems capture and reduce release of vaporized hydrocarbons to the atmosphere, for example fuel vapors released from a vehicle gasoline tank during refueling. Specifically, the vaporized hydrocarbons (HCs) are stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel. The fuel vapor recovery system may include one more check valves, ejectors, and/or controller actuatable valves for facilitating purge of stored vapors under boosted or non-boosted engine operation.
Various approaches have been developed for detecting fuel vapor leaks and/or degraded components in such fuel vapor recovery systems. However, the inventors have recognized several potential issues with such methods. The inventors have recognized that, in particular, it may be difficult to diagnose one or more valves controlling flow of purge gases from the fuel vapor canister to the intake passage upstream of the compressor (e.g., during non-boosted conditions). For example, it may be difficult to determine if a check valve positioned downstream of a compressor purge valve and upstream of an ejector and intake passage, upstream of the compressor, is stuck in an open position. If this check valve is stuck in an open position, during natural aspiration (e.g., non-boosted) operation, intake air through the open path may be sucked into the engine. This unmetered air may cause the air-fuel ratio to decrease (and become leaner than desired), thereby increasing NOx emissions. Specifically, the inventors have recognized that it may be difficult to diagnose a position of this check valve during regular boosted or non-boosted (e.g., vacuum) modes without the aid of additional sensors. However, adding sensors for this diagnosis may increase engine costs and complicate engine control.
In one example, the issues described above may be addressed by a method for adjusting one or more canister purge valves arranged in a flow passage coupled to a fuel vapor canister, an intake manifold, and an intake passage upstream of a compressor, to allow flow through the flow passage between the intake passage and intake manifold and not to the canister; and indicating the one or more canister purge valves are degraded based on a change in air-fuel ratio following the adjusting. In this way, a position of the one or more canister purge valves may be diagnosed and undesired airflow from the intake passage to intake manifold which may result in unrequested enleanment may be detected, thereby increasing engine air-fuel control and increasing engine efficiency.
As one example, the one or more canister purge valves may be a single, three-way canister purge valve arranged at a junction between a first passage coupled to the canister, a second passage coupled to the intake manifold, and a third passage coupled to the intake passage. A controller may actuate the three-way canister purge valve to move into a first position that is open to the intake passage and intake manifold and closed to the canister. As a result, intake air may be allowed to flow from the intake passage to the intake manifold. However, if the canister purge valve is degraded (e.g., doesn't move when commanded) or is stuck in a position that is open to the intake manifold and intake passage, intake air may have been flowing into the intake manifold from the intake passage via the canister purge valve both before and after the command to move the valve into the first position (and thus the air-fuel ratio may not change after commanding the valve into the first position). Thus, by monitoring a change in air-fuel ratio after commanding the canister purge valve into the first position, the controller may determine whether the canister purge valve is in the commanded position or not. This may allow the controller to diagnose a position or functioning of the canister purge valve without the use of additional sensors or control routines, thereby decreasing engine costs and control complexity. Additionally, by diagnosing a stuck or degraded canister purge valve, undesired lean engine operation may be detected and the controller may take corrective action to reduce a degradation in engine performance resulting in the lean air-fuel ratio. For example, the controller may increase fuel injection to compensate for the excess air flowing to the intake manifold via the canister purge valve. In this way, engine control may be increased and engine output may be maintained at a requested level.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.